Chlorine dioxide (ClO2) has many industrial and municipal uses. When produced and handled properly, ClO2 is an effective and powerful biocide, disinfectant and oxidizer.
ClO2 is used extensively in the pulp and paper industry as a bleaching agent, but is gaining further support in such areas as disinfection in municipal water treatment. Other end-uses can include disinfection in the food and beverage industries, wastewater treatment, industrial water treatment, cleaning and disinfection of medical wastes, textile bleaching, odor control for the rendering industry, circuit board cleansing in the electronics industry and uses in the oil and gas industry.
In water treatment applications, ClO2 is primarily used as a disinfectant for surface waters with odor and taste problems. It is an effective biocide at low concentrations and over a wide pH range. ClO2 is desirable because when it reacts with an organism in water, chlorite results, which studies to date have shown does not pose a significant adverse risk to human health at a concentration of less than 0.8 parts per million (ppm) of chlorite. The use of chlorine, on the other hand, can result in the creation of chlorinated organic compounds when treating water. Such chlorinated organic compounds are suspected to increase cancer risk.
Producing ClO2 gas for use in a chlorine dioxide water treatment process is desirable because there is greater assurance of ClO2 purity when in the gas phase. ClO2 is, however, unstable in the gas phase and will readily undergo decomposition into chlorine gas (Cl2), oxygen gas (O2) and heat. The high reactivity of ClO2 generally requires that it be produced and used at the same location. ClO2 is, however, soluble and stable in an aqueous solution.
The production of ClO2 can be accomplished both by electrochemical and reactor-based chemical methods. Electrochemical methods have an advantage of relatively safer operation compared to reactor-based chemical methods. In this regard, electrochemical methods employ only one precursor, such as a chlorite solution, unlike the multiple precursors that are employed in reactor-based chemical methods. Moreover, in reactor-based chemical methods, the use of concentrated acids and chlorine gas poses a safety concern.
Electrochemical cells are capable of carrying out selective oxidation reaction of chlorite to ClO2. The selective oxidation reaction product is an anolyte solution containing dissolved ClO2 and residual reactants. To further purify the ClO2, the ClO2 gas is separated from the anolyte solution using a randomly packed stripper column. The anolyte solution is sprayed at the top of the packed stripper column while air flows in a counter current direction. The ClO2 that is in solution exchanges from solution to air at a solution-air interface.
Suction of ClO2 gas and air from the stripper column can be accomplished using an eductor or a vacuum gas transfer pump, as described in copending and co-owned application Ser. No. 10/902,681. However, the use of a traditional eductor system to deliver a ClO2 solution directly to a pressurized water system can raise reliability concerns as described in the '681 application. A vacuum gas transfer pump can alternatively be employed. The electrolytic cells described in the '813, '398 and '681 applications can, however, have increased maintenance issues for vacuum gas transfer pumps as the ClO2 gas generation rate increases. For instance, in the high-capacity ClO2 generator described in the '813 application, more vacuum gas transfer pumps between the stripper column and the absorption loop may be needed as ClO2 gas production increases.
Unlike vacuum gas transfer pumps, which have moving parts, eductors operate on a Venturi principle where a liquid is forced through a nozzle at a high velocity to create a pressure drop without moving parts. Eductors consist of two basic components: a motive nozzle for converting pressure energy to kinetic (velocity) energy, and a suction chamber where entrainment and mixing may occur. Thus, the use of an eductor in a ClO2 gas generation system typically increases system reliability over the use of a vacuum gas transfer pump. However, the gas suction rate of an eductor depends on the differential pressure between the inlet and outlet water pressure. Depending on the end-use application for a ClO2 solution, discharge pressures varying from 0 psig to 200 psig (101 kPa to 1,480 kPa) can be encountered. As the discharge pressure varies for the end-use application, the differential pressure in the eductor will also vary and cause changes in the air suction rate. Changes in the air suction rate lead to varying concentrations of ClO2 in air instead of a desired ratio of ClO2 to air that is relatively constant. Thus, the use of an eductor system to directly feed a ClO2 solution to a pressurized water system can lead to decreased generator reliability.
Electrochemical ClO2 generators, such as those described and claimed in the '813, '398 and '681 applications, can be utilized to obtain a higher yield of ClO2 gas or ClO2 solution than those previously disclosed. This can be accomplished by applying a higher current to the electrochemical cell than those previously applied. Applying a higher current to the cell increases the rate of the selective oxidation reaction of chlorite to ClO2, which results in a higher yield of ClO2 gas. A higher yield of ClO2 gas ultimately results in a higher yield of ClO2 solution. However, when more current is applied to the electrochemical ClO2 generator cell to increase the production of ClO2 gas, more heat is generated in the electrolytic cell anolyte loop. It is known that ClO2 is unstable and capable of decomposing, in an exothermic reaction, to chlorine and oxygen. Due to this instability, an operating temperature greater than about 163° F. (73° C.) can result in potentially hazardous and less efficient operation of the ClO2 generator.
Accordingly, it would be desirable to provide a ClO2 solution generator capable of reliably operating at varying ClO2 gas generation rates. Moreover, it would be desirable to provide a ClO2 solution generator than can reliably deliver a ClO2 solution into an end-use system that is pressurized.